guile (version 1.6.7)

Guile is a library designed to help programmers create flexible
applications. Using Guile in an application allows programmers to write
plug-ins, or modules (there are many names, but the concept is essentially
the same) and users to use them to have an application fit their needs.

There is a long list of proven applications that employ extension
languages. Successful and long-lived examples in the free software world
are GNU Emacs and The GIMP.

Very popular examples of extending server applications are the Apache
projects Perl and PHP modules.

Extension languages allow users, programmers, and third-party developers
to add features to a program without having to re-write the program as a
whole, and it allows people extending a program to co-operate with each
other, without having to expend any extra effort.

Guile is a programming language
Guile is an interpreter for the Scheme programming language, packaged as a
library which can be incorporated into your programs. Your users have full
access to the interpreter, so Guile itself can be extended, based on the
needs of the user. The result is a scripting language tailored to your
application.